1. Field of the Invention
This invention relates to microwave set-on transponders, and more particularly, to the generation of accurate RF signals by means of a voltage controlled oscillator in response to the carrier frequency of received low duty cycle pulse RF signals for jamming or disabling radar systems.
2. Description of the Prior Art
There is an important requirement for the microwave systems industry to be able to accurately set a microwave voltage controlled oscillator (VCO) on to the carrier frequency of a low duty cycle pulsed carrier waveform, where the input carrier frequency is unknown apriori over a wide bandwidth, typically an octave wide. The principal reason for doing this is to be able to generate microwave power on frequency in-between reception of the pulses to jam, for example a selected radar system. Often there is a related requirement for fast acquisition of the frequency of the pulsed carrier waveform. Additionally, in some systems it is required to memorize the various carrier frequencies when a number of interleaved asynchronous pulse trains are simultaneously present.
A number of methods are currently used to meet these requirements, but they all have certain drawbacks. One method is to sweep a narrowband receiver across the band of interest, and stop (briefly) at those frequencies to measure the tuning voltage. For each such tuning voltage there is a non-linear gain function that converts the bandpass tuning voltage to the appropriate driving voltage applied to the microwave VCO. One problem with this approach is eliminating one of the sidebands. This is difficult and expensive, especially if there is a large dynamic range of input power level. To avoid the unwanted sideband, a swept YIG filter can be used. Very often the instantaneous bandwidth is wider than is suitable for the desired level of resolution, but the main problem with this approach is that the preselection filtering employs a varying magnetic field, which cannot be varied quickly, as is sometimes needed, because of the inductance. However, no matter what their fast tuning capability is, the fact that the input is a low duty cycle, forces the search sweep rate to be very slow to insure that when the bandpass scans through a particular frequency it will be there long enough to receive a pulse. This means a slow lockup. Additionally, for both approaches there are severe calibration problems. To overcome the alignment problems, very tight specifications on the components are required, which raises the cost and lowers the reliability. The most important example of this is the phenomena known as "post tuning drift". That is, if you operate the VCO in one part of the band for some time, then move to another part of the band, the different tuning voltage causes a different level of power dissipation, which in turn changes the tuning transfer function. The only good way to solve this problem is to actually go to a different type of oscillator, usually the kind whose output frequency is based on a multiple of some lower accurately set frequency. This, of course, is expensive and cumbersome. Additionally, for such narrowband swept receivers, when operating in a multiple signal environment, the receivers are often required to have a relatively high data rate. This means that pulse repetition frequency (PRF) trackers are required so that the search-sweep can be interrupted long enough for the receiver bandpass to be quickly slewed bank to the expected frequency during the predicted time slot. This time multiplexing, if really needed, is what is responsible for the quick retuning requirement and is what rules out the swept YIG filter approach, since its tuning time constant is much too long to be compatible with the typical PRF rates, and the pulse densities to be expected.
To overcome some of these problems, receivers that are instantaneously broadband are often considered in the system design. One such method is to build a continguous overlapping filter bank, with detectors on each channel. This makes for a very fast lockup system that does not need PRF trackers; however, it is a "brute force" approach that can be very costly and physically large. At the higher microwave frequencies, to maintain adequate accuracy over the same percentage bandwidth means that an enormous number of filters are required. There is also the question of the alignment of all these filters, in particular, whether there will be any "holes" in the spectrum, especially when temperature variations are considered since two adjacent filters may drift in opposite directions in response to the temperature change.
Another instantaneous bandwidth technique is the use of "Instantaneous-Frequency-Discriminators" (IFD's). These are microwave subsystems that contain rather short delay lines for which the reciprocal of the time delay gives a bandwidth on the order of the total bandwidth to be monitored. Through a series of couplers the delayed and undelayed signals are directed to four crystal detectors. Because of the frequency dependent phase differences, the power going to the crystals will split unequally. The output voltages are combined in pairs and are thus indicative of frequency. Two video voltages are available from the IFD that can be thought of as the X and Y rectangular coordinates of a vector whose magnitude is indicative of the input power to the IFD, and whose angle is indicative of the frequency. Thus, in theory these outputs can be used to set on or tune the VCO. Many of the old problems remain, however, including difficult alignment, tight specifications, and post-tuning drift. Included in this is a new and difficult problem of converting the rectangular IFD X and Y coordinates into a unique single valued voltage scale corresponding to frequency. What is required is to determine the function of Arc-tan (Y/X). To accomplish this, analog hybrid divider modules, along with circuitry that senses the quandrant, along with considerable extra circuitry is employed. Needless to say, this long serial string of components and modules is not conducive to accuracy. Investigations have been conducted aimed at performing the conversion digitally, but in either case the circuitry becomes quite complicated. The other alternative is just to use a portion of two of the quadrants--say the X-axis output. In this method it is hoped that the output amplitude should indicate frequency. This, of course, lowers the overall resolution capability of the IFD since two of the quadrants are wasted. Beside that, as stated above, the input power level influences the output amplitude, so the input must be very precisely limited. This is difficult, especially if, as is the case, the input pulse widths are on the order of the servo response times that are needed to do the limiting accurately enough.
Still another way to employ the IFD's is to employ a bank of them, each with 4X the resolution of the previous one and then sense which quadrant the output is in from each IFD. This has the advantage that only the polarity of each voltage need be sensed. There are crossover problems in doing this, however, that can lead to very large errors at certain transition points in the band. This can be overcome by employing still more IFD's properly offset to resolve the conflicts, but again this reduces to the brute force approach which quickly escalates the cost, although operationally it usually works fairly well. On the plus side, the IFD approaches give a fast lockup capability, and they do not need PRF trackers since they are instantaneously broadband. One drawback is that if pulses come in simultaneously, they will be read incorrectly; in some applications this is a problem.
Another whole category of transponder designs is to take the above-mentioned basic design alternative and introduce feedback. Although this may appear to make the system block diagram more complex, in actual practice the hardware will be a lot simpler, the component specifications can be looser, and is preferred in general. This usually eliminates the post tuning drift problem, particularly if the set-on is continually updated. If we apply this to the swept narrowband receiver approach, a number of the problem areas disappear, and indeed the cost of the extra circuit complexity is more than offset by the looser specs giving lower costs. There is the additional human factor benefit in that the risk that a technician or other field personnel will misadjust the critical alignment is largely eliminated. The system will still be plagued with a long acquistion time, and the problem of an unwanted sideband of an IF strip is avoided. A PRF tracker may still be required to separate pulse trains for carrier separations that are within the discriminator pull-in range, for general data update, and possibly a number of other reasons.
Using feedback in the filter bank approach eliminates the VCO tuning transfer function alignment problem, but does nothing for the interchannel alignment of the bank itself, and this approach is therefore generally discarded. Applying the feedback approach to IFD schemes also sees many of the alignment problems melt away; however, there is still the difficult problem of making the rectangular to polar conversion, including sensing the proper quadrant, although the requirements on this function are also somewhat eased since it can be thought of as part of the VCO tuning transfer function. Thus, rather tight tolerances on a long cumbersome serial chain of modules is still required. This approach, however, still has the benefit of fast lockup, and its independence of any PRF jitter problems.
Thus, in reviewing the most common transponder design approaches, it is evident that a simple low cost self-calibrating microwave transponder or set-on approach is still needed.